This invention relates to a process of producing electrodes for vacuum circuit breakers and, more particularly, to an improvement on a process of producing the electrodes through mixing raw materials of powder and sintering the resultant mixture.
The invention is suitable in use for production of Cr-Cu base electrodes, for example, which contain Cr as a main component and Cu. The Cr-Cu base electrodes for vacuum circuit breakers can be widely used, for example, for changeover switches for vehicles, vacuum circuit breaker for wide use, etc.
It is known that vacuum circuit breaker electrodes which are constructed of conductive metal of Cu or Ag and refractory metal having a higher melting point than the conductive metal have a high withstand voltage and are suitable for interruption of a large amount of electric current. As the refractory metal, for example, Cr, Co, Ni, Fe, Ta, W, Mo, etc. are used and, in particular, Cr of those metals is used most widely.
As production methods of vacuum circuit breaker electrodes, a melting method of electrode production through melting raw materials and solidifying it to form an alloy for electrodes, or a sintering method of electrode production through sintering raw material powder is generally used. Usually, the sintering method is used for such electrode material that is low in solubility and difficult to be made into an alloy, such as a combination of Cu and Cr, or for such electrode material that is separated into two components when melted, such as a combination of Cu and Fe, a combination of Cu and Co, etc.. Japanese Patent Laid-Open No. 50-55870 discloses details about production of electrodes consisting of conductive metal and refractory metal by sintering.
Of many methods of producing vacuum circuit breaker electrodes by sintering, a method of mixing and compacting raw materials of powder and then sintering the compact, which is similar to a method described in the Japanese Patent Laid-Open No. 50-55870, is the main method current.
The production of electrodes by a sintering method is always accompanied by a problem of oxidation. The Japanese Patent Laid-Open No. 50-55870 proposes sintering in a high vacuum or in a reducing atmosphere as measures for preventing the oxidation.
The inventors confirmed that electrodes which consist of conductive metal and refractory metal and are produced by sintering have large variation in withstand voltage. Even if the raw materials of powder are degassed in advance, or the sintering is effected in vacuum or in a reducing atmosphere, the variation in withstand voltage could be almost never improved. From these facts, it is found that the electrode production technique using the conventional sintering methods is not suitable for a method of producing electrodes having high withstand voltage.
The Japanese Patent Laid-Open No. 50-55870 does not discloses anything about withstand voltage characteristics and suggests nothing about relationship between sintering methods and the withstand voltage.